Lithography process is usually used in a semiconductor process, which transfers the patterns of masks to a wafer in layer by layer and then a chip is formed later. If the mask has a defect, the defect is also transferred to the wafer, so the mask inspection, before manufacturing semiconductor, is applied to avoid transferring the defect of the mask. Lots of mask inspection techniques have been developed.
The lithography technique keeps developing and getting smaller, and now the nano-level technology has been developed, such as the extreme ultraviolet (EUV), but the technology is not ready, because the nano-level mask inspection technique and tool are not prepared and valid. In nano-level scale inspection of the mask, EUV reticle, the conventional optical inspection technology can not satisfy the requirement of the high resolution generally. To increase the energy of X-ray (high frequency X-ray) can improve the resolution. However, high energy X-ray inspection may damage the EUV reticle. Some proposed approaches of nano-level mask inspection technique are discussed as follows:
In US20080318138, one side of the mask is coated by a reflection layer and the other side is coated by a conductive layer. To detect the transmitted light and the reflected light and then constitute an image of the EUV reticle to enhance the contrast of a detected image of the EUV reticle. That can enhance the accuracy of inspecting the EUV reticle, but make the process of manufacturing an EUV reticle more complex.
In U.S. Pat. Nos. 7,440,093 and 7,271,891, a susceptibility material is formed on the reticle and then the defects (if existed) will has different susceptibility. The susceptibility of different areas of the EUV reticle should be different and then a susceptibility map of the EUV reticle could be reconstruct by the different susceptibility of different areas, so the defects of the EUV reticle can be inspected. That inspects the EUV reticle indirectly but directly and the susceptibility material of the EUV reticle also makes the process of manufacturing an EUV reticle more complex.
In US20070064997, two separate inspection steps are used. An optical inspection is firstly used to inspect the EUV reticle for finding out the defect areas in a larger scale, and the optical inspection has a rough resolution. Then an electron beam (EB) inspection is used to inspect the defect areas to find out the precise defect with high resolution of inspecting the defect of the EUV reticle. Two inspection tools are used in the prior art to have a complex inspection process, and the energy of the EB inspection may be too large to damage the EUV reticle. For example. The energy of an EB used in inspecting the EUV reticle is high to 50KV, and so high energy will damages the EUV reticle. Besides, the surface charge of the EUV reticle is not processed in this prior art, which may distort the result of the inspection to reduce the accuracy thereof.
In U.S. Pat. No. 7,090,948, a buffer layer is disposed on a hard mask. The buffer layer is transferred into a hard mask in an etching process. In an initial inspection, defects are found and the then the hard mask can be transferred into the reflection layer for repairing the EUV reticle. The invention provides a mechanism of repairing a mask but not the mechanism of inspecting the EUV reticle. Although accompanying with the atomic force microscope, the skill is used to check the alignment of the EUV reticle, which does not inspect the EUV reticle.
In U.S. Pat. No. 7,407,729, a buffer layer with hard magnetic material is coated on a Bragg reflector on substrate of an EUV reticle and an absorber layer with soft magnetic material is coated on the buffer layer. Finally an imaging process is made with respect to the magnetic properties of the EUV reticle. Accordingly that makes the process of manufacturing the EUV reticle more complex, and the cost thereof is increased.
In U.S. Pat. No. 6,620,563, an atomic force microscope (AFM) is used to check the alignment of the EUV reticle, which also does not inspect the EUV reticle.
In U.S. Pat. No. 7,179,568, a dye material is incorporated in a photoresist layer and then exposed under radiation source, such as EUV or X-ray (Cu k-α(.alpha.)). The incident beam and the reflected beam will be combined to form an exposure curve in an intensity-position system. The exposure is insufficient at the positions of the defects in a critical exposure period, and the exposure curve in the intensity-position system will be reduced and less than a critical threshold value. The exposure below the critical threshold will be related to the defects of the EUV reticle, and the corresponding positions are located. Similar with the mentioned-above prior arts, a dye material and a photo-resist layer are introduced and that increases the complexity of manufacturing the EUV reticle.
In U.S. Pat. No. 6,555,828, an about 13 nm EUV is projected and three possible phenomena will can occur, an intensity reduction, an off-specular direction and a change of amplitude and phase of the electric field at the surface. These effects will reduce the reflected photoemission beam, which causes a reduction of a related EUV light, and the defect of the EUV reticle can be determined by observing the reduction of EUV light.
In US20020035461, an algorithm is used to find out defects according to an inspection of an EUV reticle. The algorithm calculates and defines an image and then compares the ideal image to find out the defects. That uses an algorithm to find the defect but not to directly inspect the defects of the EUV reticle.
Other prior arts inspect the defects of an EUV reticle by using EUV LASER pulse, and these arts have thermal and resolution issues usually.
According to the above discussion, a direct, efficient, simple and accuracy inspection of an EUV reticle is an important topic. The present invention proposes a method and an apparatus for inspecting the defects of the EUV reticle, the method and apparatus have these advantages, and the detailed is as follows.